1. Field of the Invention
The present invention relates to a storage system, and in particular, a technology that speeds up a storage system and improves reliability.
2. Description of the Related Art
In a storage system (disk array device), an increase in capacity of a storage module such as a hard disk drive (HDD) or a Solid State Drive (“SSD” or “semiconductor memory storage device”) leads to an increase in capacity of a group (a RAID group or a storage pool) having the storage module.
In many cases, an upper limit capacity of a logical volume (LU) on the group having the storage module is 2 TB. The reason for the upper limit capacity is that an LBA specified area as parameters of the READ (10) command and the WRITE (10) command in the SCSI protocol normally used in a block access-type storage apparatus is four bytes. As a command with an LBA area expanded, variable-length READ (16) command and WRITE (16) command are defined, but many storage systems do not yet support those commands.
A plurality of LUs may be in the same group. In this case, even if access is performed to logically different LUs, the access is actually performed to the physically same storage devices. The access to the same storage devices invites a bottleneck in view of performance.
A hard disk drive, which is popularly used as a storage device, has a response time cause of disk rotation or a head seek time in the unit of millisecond. For this reason, performance depending on a mechanical cause does not follow the improvement in performance of an interface or a controller. That is, the mechanical cause becomes a bottleneck of performance.
A Solid State Drive (SSD) does not have a mechanical response time, and thus data may be read and written at high speed. However, the SSD is designed focusing on compatibility with the hard disk drive, it is difficult to sufficiently use read and write performance of a semiconductor. For example, if an SAS controller has eight ports, and each SAS interface may transfer data at 6 Gbps, the total bandwidth is 8×6 Gbit/s=48 Gbit/s. The SAS has a function to group a plurality of links and transfer data through the links (called wide port or wide link). Thus, it is possible to sufficiently use the bandwidth.
In regard to the method in which a plurality of physical links are grouped and data is transferred through the physical ports, JP-A-2007-256993 discloses a storage system in which an SAS initiator controls how many physical links of a wide link are allocated to an SAS target. U.S. Patent Publication No. 2008/0104264 discloses a technology that transfers data through a wide port formed by grouping a plurality of physical links.